In the games

Serena asking to battle with everyone.

Serena is a girl living in Vaniville Town with her friends Tierno, Trevor, Shauna, and her next door neighbor Calem. The five of them form a bond almost instantaneously and they decide to go on their Pokémon adventures together.

Trivia

Prior to the release of Pokémon X and Y, pre-release media referred to Serena as "Yenna."